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Full text of "Treatise On Applied Analytical Chemistry(Vol-1)"

CALCIUM CITRATE

61

precipitate added and gentle heat again applied. A distinct wine-red
coloration indicates the presence of tartaric acid.

6. Determination of the Citric Acid.—10 grams of the citrate are
boiled gently in a graduated 250 c.c. flask with 22 c.c. of hydrochloric acid
of D i-io and about 50 c.c. of distilled water to expel the carbon dioxide
completely. When cold, the liquid is made up to the mark, shaken and
filtered through a dry paper. 50 c.c. of the filtrate (=2 grams of the
citrate) are neutralised exactly with approximately 2N-caustic soda free
from carbonate, using phenolphthalein as indicator. After addition of
2 c.c. of roughly 40% calcium chloride solution, the liquid is made faintly
acid with a few drops (4-6) of N/2-hydrochloric acid.

This liquid, in a beaker of resistant glass, is kept for half an hour im-
mersed in a bath of boiling brine and then filtered hot through a rapid
filter, on to which the calcium citrate precipitate is washed as completely
as possible with hot water ; the precipitate, is washed with boiling water,
not more than 150 c.c. being used altogether. The precipitate (I) is then
dried in a steam-oven, while the filtrate is neutralised with a few drops of
dilute ammonia (i : 6) and concentrated to 30-40 c.c. in the beaker pre-
viously used for the precipitation. The liquid is then placed in a smaller
beaker (50 c.c.), another drop of ammonia being added and the concentra-
tion continued to 15 c.c. The precipitate is then collected on a small filter
and rapidly washed with small quantities of boiling water.

This precipitate (II) is dried and the filtrate, treated as before with a
little dilute ammonia, concentrated by boiling to 10 c.c. The precipitate
(III) is collected on a small filter, washed with very small amounts of boiling
water and dried in the steam-oven.

The three dry precipitates are incinerated with the filter-papers in a
platinum dish and the latter kept in a muffle at redness for 30 minutes.
The ash is then treated with 50 c.c. of N/2-hydrochloric acid, which is added
to the dish in small portions, and then transferred to a flask. The liquid
is boiled carefully to dissolve the ash completely, then cooled and the excess
of acid titrated with N/4-potassium hydroxide in presence of phenolphtha-
lein. The quantity of citric acid is then calculated ; I c.c. of N/4-alkali =
0-0175 gram of crystallised citric acid or 0-875% on the sample.

This method has been officially adopted in Italy for the analysis of crude
citrate and concentrated lemon, juice.

Where marked quantities of tartrate have been found, it is advisable not to
concentrate the solutions much for collecting precipitates II and III—instead
of 15 and 10 c.c. only to 20 and 15 c.c. respectively.

In case sulphates are present, the ash from the precipitates should be treated
with 10 c.c. of 3% hydrogen peroxide solution, which is then slowly evaporated
on a water-bath, the treatment with 50 c.c. of N/2-hydrochloric acid being after-
wards carried out as described above.

*
#  *

Crude calcium citrate contains 64-70% of crystallised citric acid (+ iH2O)
in combination with lime ; usually the percentage varies between 64 and 66,
but impure samples containing 59-63% are not rare.

The alkalinity of the crude citrate, deduced from the free lime and calcu-
lated as calcium carbonate, should not exceed 2%, otherwise the price is subject
to reduction.cid, evaporated on aly for delicate colours, should not contain iron.                                                                       l.2o